September 25, 1997

Pushbutton switch controls
power supply and µC

Greg Dittmer, Linear Technology Corp, Milpitas, CA

Switching handheld units on and off with a pushbutton switch is a desirable feature. Thus, switching-regulator/ controller ICs for battery-powered applications provide a logic input to switch the device to a shutdown mode. This type of shutdown pin can require additional components and logic gates to implement the pushbutton interface. However, a simple circuit can implement an on/off function with no external logic (Figure 1). The circuit is based on IC₁'s step-down controller, which provides the toggle on/off function internally with a SET/RESET latch. Momentarily grounding the ON pin sets the latch and turns on the regulator; momentarily grounding the LBI/OFF pin resets the latch and turns off the regulator.

In Figure 1, with both the µC and the switching regulator powered down, pressing the pushbutton switch grounds IC₁'s ON pin and starts the regulator, which then powers up the µC. When you press the pushbutton switch a second time, the µC input detects the pressed switch's state. The µC then pulls the LBI/OFF pin low through a connection to one of its outputs. With the LBI/OFF pin low, IC₁ powers down, turning off the µC.

The diode between IC₁'s ON pin and the µC is necessary because most µCs have a parasitic diode between each input and the supply. The '914 is thus necessary to prevent the µC, when it powers down, from pulling down IC₁'s ON and subsequently turning back on the regulator. (DI #2094)

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